Mechanical allodynia following contusion injury of the rat spinal cord is associated with loss of GABAergic inhibition in the dorsal horn

The present study investigated whether mechanical allodynia following contusive spinal cord injury (SCI) of the thoracic segments 12 and 13 of the rat was associated with a reduction in gamma-aminobutyric acid (GABA)ergic inhibition adjacent to the site of injury. Five to 7 days following SCI, extracellular recordings were obtained from dorsal horn neurones located 1-2 segments caudal to the injury, in non-allodynic and allodynic halothane anaesthetised rats and from comparable neurones in normal rats. To assess spinal GABAergic inhibition in the three groups of animals, spontaneous and evoked cell firing rates were recorded before, during and after microiontophoretic application of the GABA(A) receptor antagonist bicuculline. Administration of bicuculline to normal animals resulted in significant and reversible increases in the receptive field size, spontaneous firing rate, response to brushing and pinching the skin and afterdischarge activity of dorsal horn neurones, as well as decreasing paired-pulse depression of responses evoked by transcutaneous electrical stimulation. In non-allodynic SCI animals, bicuculline ejection led to significant changes in receptive field size, paired-pulse depression and responses to brush and pinch stimulation that were comparable to those observed in normal animals. By contrast, in allodynic SCI animals, bicuculline ejection had little or no effect on dorsal horn neurone responses to mechanical skin stimuli and paired-pulse depression despite reliably blocking the inhibition of cell firing produced by similarly applied GABA. The demonstration of reduced GABAergic inhibition predominantly in the allodynic SCI rats suggests that such a deficiency contributed to this pain-related behaviour acutely following SCI.